1. Field of the Invention
The present invention relates to a method of producing a granular amorphous silica and, more specifically, to a method of directly producing a spherical amorphous silica in a step of neutralizing a mixture solution of an alkali silicate and a carboxymethyl cellulose (CMC) with an acid.
2. Prior Art
Owing to their properties, amorphous silica particles have been extensively used as a filler for paints, information-recording papers, rubbers and resin-molded articles and as a carrier for catalysts, chromatos, perfumes, medicinal components and the like. They can be produced by a dry method or a wet method. The dry-method silica particles have a fine spherical shape as they are obtained by decomposing SiCl.sub.4 in an oxygen-hydrogen flame and have relatively small surface activity due to their specific surface area, porous volume and porous distribution. The wet-method silica particles have relatively large surface activity and are indefinite in shape as they are obtained by neutralizing an alkali silicate with an acid. In particular, the latter wet method makes it possible to obtain an amorphous silica having widely different properties by changing the conditions such as the concentration of the reaction product, temperature, pressure, time and reaction method at the time of neutralizing the alkali silicate with an acid.
In utilizing the amorphous silica particles for a variety of applications, one of the important characteristics is the dispersion property of the silica particles. To obtain this property, the particles must have a predetermined spherical shape with small coagulating property.
So far, fine spherical silica particles have been produced by hydrolyzing an organosilane in an organic solvent such as ethanol, by forming a silica sol or gel into spheres, by preparing a W/O emulsion from an aqueous solution of alkali silicate and an organic solvent followed by hydrolysis, by adding an organic gelating agent such as formamide to a mixture of a colloidal silica and an alkali silicate by forming fused silica into spherical particles, and by treating a definite shape particles of various kinds of zeolites with an acid. According to the above conventional methods, however, the starting materials that are used are expensive, and the aforementioned demands are not satisfied to a sufficient degree.
In recent years, U.S. Pat. No. 4,752,458 is disclosing a method of producing fine spherical silica comprising adding an acid solution to a solution of a soluble silicic acid, and then adding thereto a metal salt of alkali alginate, ammonium alginate, starch, gelatine, pectin, or an organic polymer solution of a mixture thereof prior to forming a gel.
The present inventors have proposed in Japanese Laid-Open Patent Publication No. 193927/1993 a method of producing amorphous spherical silica particles by adding an acrylamide-type water-soluble high molecular polymer in a step of neutralizing an alkali silicate with an acid.
The above prior arts are excellent in their idea for directly producing fine spherical silica particles by adding a water-soluble high molecular polymer in a step of neutralizing the alkali silicate with an acid, but leave problems that must be solved in practicability.
First, according to the former method, the yield of the fine spherical particles is about 40%, the obtained particles have uneven shapes and particle sizes, and are very poorly filtered exhibiting very poor practicability (see Comparative Examples appearing later).
According to the latter method, on the other hand, the yield of fine spherical particles is greatly improved, the obtained particles exhibit uniform shapes and particle diameters and are further excellently filtered accompanied, however, by a problem in that a coagulation growing agent which is used is a relatively expensive water-soluble high molecular polymer of the type of acrylamide and must be used in large amounts still leaving room for improvement from the standpoint of cost.